This invention relates to an organic electroluminescent (EL) device using an organic compound and a method for preparing the same, and more particularly, to an electron injecting electrode and a method for preparing the same.
Recently, active research works have been made on organic EL devices. As a basic configuration, the organic EL device includes a transparent electrode or hole injecting electrode of tin-doped indium oxide (ITO) etc., a thin film formed thereon by evaporating a hole transporting material such as triphenyldiamine (TPD), a light emitting layer deposited thereon of a fluorescent material such as an aluminum quinolinol complex (Alq3), and a metal electrode or electron injecting electrode formed thereon from a metal having a low work function such as magnesium. Such organic EL devices are attractive in that they can achieve a very high luminance ranging from several 100 to several 10,000 cd/m2 with a drive voltage of approximately 10 volts.
With respect to the material used as the electron injecting electrode of such organic EL devices, a material capable of injecting more electrons into the light emitting layer or the electron injecting and transporting layer is believed effective. Differently stated, a material having a lower work function is believed suitable as the electron injecting electrode. There are known many materials having a low work function. For use as the electron injecting electrode of organic EL devices, JP-A 15595/1990, for example, discloses electron injecting electrodes constructed of plural metals other than alkali metals wherein at least one of the metals has a work function of less than 4 eV, such as MgAg.
Alkali metals are preferred materials having a low work function. U.S. Pat. Nos. 3,173,050 and 3,382,394 describe Na and K as exemplary alkali metals. Since electrodes using alkali metals, however, are highly active and chemically unstable, they are inferior in safety and reliability to electron injecting electrodes using MgAg or the like.
Known attempts to enhance the stability of electron injecting electrodes using alkali metals include the electron injecting electrodes of AlLi alloys described in JP-A 165771/1985, 212287/1992, 121172/1993, and 159882/1993, for example. Reference is made to the lithium concentration and preparation of the AlLi alloys described in these patent publications. (1) JP-A 165771/1985 discloses a lithium concentration of 3.6 to 99.8 at % (1 to 99 wt %), preferably 29.5 to 79.1 at % (10 to 50 wt %), with AlLi alloys containing 15.8 to 79.1 at % (4.8 to 50 wt %) of lithium being described in Examples. All these AlLi alloys were deposited by an evaporation process. (2) JP-A 212287/1992 describes a lithium concentration of at least 6 at %, preferably 6 to 30 at %, with an AlLi alloy containing 28 at % of lithium being described in Example. It is also described that these AlLi alloys can be deposited as films by resistive heating co-evaporation, electron beam evaporation and sputtering, although only the evaporation process is used in Examples. (3) JP-A 121172/1993 discloses a lithium concentration of 0.0377 to 0.38 at % (0.01 to 0.1:100 weight ratio), while AlLi alloys containing 0.060 to 0.31 at % (0.016 to 0.08:100 weight ratio) of lithium are formed by resistive heating evaporation or electron beam evaporation in Examples. It is described that the preferred concentration is up to 15.9 at % (up to 50:1000 weight ratio), while AlLi alloys containing 29.5 to 61.8 at % (10 to 30 wt %) of lithium are formed as films in Examples. (4) JP-A 159882/1993 discloses a lithium concentration of 5 to 90 at % and describes in Examples that AlLi alloys having a lithium concentration of 16 to 60 at % are formed into films by a two-source evaporation process using resistive heating evaporation for the lithium source and electron beam evaporation for the other source.
However, the AlLi alloy electrodes of (1), (3) and (4) described above are formed into films solely by vacuum evaporation. As to the AlLi alloy electrode of (2), a sputtering process is referred to, but a vacuum evaporation process is used in Examples and no exemplary sputtering process is described.
Where the vacuum evaporation process is used, an AlLi alloy is used as the lithium evaporation source since lithium alone is inferior in chemical stability, film formability and adhesion. However, because of different vapor pressures of the two metals; two-source evaporation or co-evaporation along with aluminum is necessary. The two-source evaporation is not easy to control the composition of a deposit and difficult to consistently provide an optimum composition in every run. As a consequence, the lithium concentration of actual deposits is biased to a relatively high concentration range of 16 to 79 at % and remains inconsistent. Higher lithium concentrations entail chemical instability and eventually, cause to exacerbate the film formability and adhesion of deposits, deteriorating device characteristics. The deposits are inconsistent in quality too. Conversely, if evaporation is carried out from a single evaporation source, the lithium concentration becomes as low as 0.38 at % or less. Such alloys have a high work function and hence, a low efficiency of electron injection, making it difficult to produce devices having practically acceptable characteristics.
Also, the electron injecting electrodes formed by the vacuum evaporation process are less dense as a film and less adhesive to the organic layer at the interface, which cause to exacerbate the performance, lifetime and display quality of EL devices by way of a drop of light emission efficiency, peeling of the electrode and the concomitant development of dark spots.
Further, since lithium or a similar material having a low work function is highly reactive to oxygen and moisture and the steps of feeding and supplementing the material are generally carried out in the air, oxide forms on the material surface. To form electron injecting electrodes of quality, it is preferred to remove the oxide coating prior to evaporation. Removal of the oxide coating is difficult since it seldom occurs that the oxide has a lower evaporation temperature or a higher vapor pressure than the elemental metal. It is thus not easy to form a high quality electron injecting electrode in the form of a pure metal film. Furthermore, if an evaporated film of such an oxide is formed at the interface between the electron injecting electrode and the organic layer or within the electron injecting electrode, desired EL characteristics are not obtainable because the resulting electrode has a different work function and electric conductivity from those of the elemental metal. Further, many problems concerning productivity arise from the practical point of view; for example, there arise problems with respect to composition control and the uniformity of film thickness and quality if exchange or supplement of the material to be evaporated becomes necessary within a short period or the deposition area is increased; and there arise problems with respect to composition control and the reproducibility and uniformity of film quality if the rate of deposition is increased.
An object of the invention is to realize an organic EL device featuring a high luminance, high efficiency, long lifetime, and high display quality, by improving the film formability and adhesion at the interface between an electron injecting electrode and an organic layer.
This and other objects are achieved by the construction defined below as (1) to (4).
(1) An organic electroluminescent device comprising a hole injecting electrode, an electron injecting electrode, and at least one organic layer disposed between the electrodes,
said electron injecting electrode being comprised of an AlLi alloy containing 0.4 to 14 at % of lithium deposited by a sputtering process.
(2) A method for preparing an organic electroluminescent device, wherein an electron injecting electrode as set forth in (1) is deposited by a sputtering process using an AlLi alloy as the target.
(3) A method for preparing an organic electroluminescent device according to (2) wherein the sputtering process involves changing the pressure of film forming gas in the range of 0.1 to 5 Pa for changing the concentration of lithium in the electron injecting electrode being deposited in the range of 0.4 to 14 at %.
(4) A method for preparing an organic electroluminescent device according to (2) wherein the sputtering process is a DC sputtering process.